1. Field of the Invention
The present invention relates to a digital modulation/demodulation method and system suitable for use in a mobile communication system and also in a digital communication device.
2. Description of the Related Art
In recent years, .pi./4-shift quadrature phase shift keying (QPSK) has developed as a digital modulation technique having high resistance to disturbance such as fading and thus is suitable for use in a mobile communication system. The .pi./4-shift QPSK technique is a QPSK technique in which a carrier signal takes one of four phase shift states assigned to 2-bit digital data thereby transmitting desired information.
Referring to FIGS. 10a and 10b, signal locations in QPSK and .pi./4-shift QPSK will now be described. FIGS. 10a and 10b are signal space diagrams illustrating modulated signals in polar coordinates for QPSK and .pi./4-shift QPSK. In these figures, the vertical axes Q and the horizontal axes I represent the quadrature and inphase components, respectively, of the modulated signal. Hereinafter, points plotted in the signal space diagram are referred to as signal points.
In QPSK, there are four signal points having a phase difference .+-..pi./4 or .+-.3.pi./4 relative to the carrier signal, wherein these four points correspond to respective 2-bit digital data. That is, in QPSK, there are four different symbols or discrete states which can be taken by a signal during a predetermined time period. Furthermore, in QPSK, the phase difference between successive symbols can be 0, .+-..pi./2, or .pi.. For example, in FIG. 10a, when a symbol is assumed to be at a signal point a, at a certain symbol time, the transitions denoted by arrows in the figure are possible depending on the data to be transmitted. That is, (i) if the data to be transmitted at the following symbol time is equal to "00", then a transition to the signal point a occurs (no change occurs in the phase in this case); however, (ii) if the data to be transmitted at the following symbol time is equal to "01", then a transition to the signal point b occurs; (iii) if the data to be transmitted at the following symbol time is equal to "10", then a transition to the signal point c occurs; and (iv) if the data to be transmitted at the following symbol time is equal to "10", then a transition to the signal point d occurs; and thus corresponding changes in phase occur.
On the other hand, in .pi./4-shift QPSK, the phase difference between successive symbols can be .+-..pi./4 or .+-.3.pi./4. For example, in FIG. 10b, if a symbol is at a signal point a' at a certain symbol time, the transitions denoted by arrows in the figure are possible depending on the data to be transmitted. That is, if the data to be transmitted at the following symbol time is "00", then a transition to the signal point a' occurs. Similarly, a transition to the signal point b' occurs for the symbol "01", to the signal point c' for the symbol "11", and to the signal point d' for the symbol "10", and thus corresponding changes in phase occur.
In .pi./4-shift QPSK, as described above, transitions occur at each symbol time alternately between points denoted by .smallcircle. and .circle-solid. in FIG. 10b. This means that although .pi./4-shift QPSK includes eight apparent signal points in the signal space, the phase states which can be taken at the subsequent symbol time are limited to four states and thus each symbol transmits 2-bit information.
In the .pi./4-shift QPSK technique described above, as can be seen from FIG. 10b, any transition occurs without passing through the origin of the signal space. This results in a reduction in the variation in the envelope amplitude of the modulated signal, and therefore, it is possible to suppress nonlinear distortion which would otherwise occur in power amplification in a communication device. In the .pi./4-shift QPSK technique, unlike the QPSK technique, even when the same data is transmitted successively, a phase change of .pi./4 radians occurs for each transmission of the data. This means that the phase of the modulation signal always changes and thus the .pi./4-shift QPSK technique has the advantage that the timing can be easily extracted from the signal.
Amplitude phase shift keying (APSK) is also a known digital modulation technique. In APSK, two parameters, namely the amplitude and phase of a carrier signal are modulated in accordance with the value of digital data to be transmitted. For example, signal locations for 16-QAM (quadrature amplitude modulation) are shown in the signal space diagram of FIG. 11. As can be seen, in multilevel digital modulation techniques such as 16-QAM in which both phase and amplitude are modulated, symbols (denoted by .circle-solid. in FIG. 11) are located in the signal space such that they are uniformly spaced by an equal Euclidean distance.
Such signal locations are obtained, for example, if sinusoidal signals which differ in phase by 90.degree. from each other are subjected to 4-level ASK modulation according to 2-bit data, respectively, and if the resultant two modulated signals are added together. Thus, in 16-QAM, each symbol is represented by 4-bit data. As a result, 16-QAM can transmit information at a high data transmission rate compared to QPSK and .pi./4-shift QPSK.
In multilevel digital modulation techniques such as 16-QAM, in order to make it possible to determine the levels of symbols having an equal phase, it is required to place preamble signals (denoted by .smallcircle. in FIG. 11) at predetermined time intervals so that the amplitude of the received signal can be evaluated relative to the amplitude of the preamble signals.
A problem of the .pi./4-shift QPSK is that when data is transmitted at a high transmission rate, the occupied bandwidth increases with the transmission rate, and thus it becomes impossible to use the frequency band in an efficient fashion. To achieve high-speed data transmission, if APSK (amplitude phase shift keying) is performed in a multilevel fashion, a greater dynamic range of the signal amplitude is required. The increase in the dynamic range can result in nonlinear distortion when the signal is amplified by a transmission power amplifier. To avoid such distortion, it is required that the power amplifier should have good linearity over a wide dynamic range. However, such a power amplifier needs a greater operating current and thus the power efficiency becomes poor.
Furthermore, the multilevel APSK signal has a small Euclidean distance among symbols. That is, when symbols are plotted in a polar-coordinate signal space, they are spaced by a small distance. This means that the multilevel APSK signal has poor resistance to fading and noise, and thus an additional circuit is required at a receiving end to compensate for such effects, especially when the multilevel APSK technique is employed in applications in which communication devices move at a high speed. As can be understood from the above discussion, although the multilevel APSK can be employed to achieve high-speed data transmission in an environment where good radio wave propagation is obtained, the error rate becomes high when it is employed in a bad environment in terms of the radio wave propagation in which disturbance such as multipath fading occurs, as is the case in mobile applications. The increase in the error rate causes the effective transmission rate to decrease to a level nearly equal to or even lower than the low transmission speed obtained in .pi./4-shift QPSK.
Furthermore, if preamble signals are inserted, as described above with reference to 16-QAM, in a transmission signal at fixed time intervals so that the amplitude of the received signal is evaluated with reference to the amplitude of the preamble signals, a mobile communication system employing such a technique will encounter the following problems:
(1) When a great variation occurs in the intensity of a radio wave within a preamble signal interval, there will be a high possibility that an error occurs in detection of ASK signals (refer to FIG. 12, in which shaded regions denote preamble signals);
(2) If the preamble signal interval is set to a smaller value so as to suppress the influence of fading which occurs at short time intervals, then the amount of information which can be transmitted decreases by a corresponding amount (refer to FIG. 13).